The discrete cosine transform (DCT) and the discrete wavelet transform (DWT) are two important transforms used in image/video coding applications. It is observed that the wavelet based image encoding scheme outperforms other type of coders in terms of reconstruction quality as well as coding efficiency. To achieve efficient bandwidth and buffer utilization by wavelet coding and adding the flexibility of DCT based services, there is a requirement of intermediate transcoder. Transcoding also provides inter-operability and brings heterogeneous devices together in a multimedia-multiuser environment. The design of transcoder in the transform domain is of importance as it avoids full de-compression and re-compression operations. In this thesis, a novel separable linear filtering is also proposed using symmetric convolution in the DCT domain. New algorithms are proposed for transcoding DCT coefficients into wavelet subbands and vice versa by using filtering techniques in the DCT domain. In the proposed approach, the computations are performed in the transform domain, so as to obtain the same result as obtained by the spatial domain approach. For DWT to DCT transcoding, the wavelet coefficients are first transformed into upsampled DCT coefficients. Subsequently, these transformed coefficients are synthesized in the block DCT space. To reduce the cost of transcoding, upsampling and filtering operations are performed in a single combined step. Similarly, the DCT to DWT transcoding is based on the combined step of block filtering and downsampling directly in the DCT domain. Here, the combined operation of filtering, IDCT and downsampling, significantly lower the computational requirements. Typical sparseness of DCT domain input blocks is considered for further reduction of complexity. The proposed techniques use simple linear operations such as matrix multiplication, which is appropriate for efficient hardware implementations. In this thesis, resizing of images in the block DCT domain is also proposed. As an application, reformatting of JPEG2000 to JPEG using the wavelet to DCT transcoding technique is presented. Transcoding also helps in adapting processing with wavelet coefficients given the input in the block DCT. A typical example of deblocking DCT coefficients is also presented in the proposed work. Keywords– Transform domain processing, DCT, DWT, Transcoding, Symmetric convolution, Block filtering, Linear filtering, DCT domain filtering, Resizing, Deblocking.